If they were presented with an object they didn’t like, such as pair nail clippers, the animals moved their right ear instead.

Dr Miho Nagasawa, from the department of animal science at Azabu University in Sagamihara, Japan, said: “It is difficult to explain this difference in movement between the ears and eyebrows. “Dogs’ ears are prominent features used to convey emotional expression, therefore our results suggest that dogs were more cautious toward unfamiliar people.

“In contrast, eyebrow movement might indicate a visible response where by dogs attempted to look at their owners more intently.”

The researchers, whose study is published in the journal Behavioural Processes, placed 12 dogs in a room divided by a partition with some black curtains that opened briefly to all the dogs to glimpse what was on the other side.

Coloured tags were placed on the dog’s faces to allow a high speed camera to precisely track the movement of their features.

When the curtain opened, their owner, a stranger, a toy or an item they did not like was on the other side, allowing the researchers to record their response.

They found that the dogs moved their eyebrows upwards whenever they saw someone behind the screen, but far more when it was their owner, particularly on the left side.

The researchers suggest the subtle changes on different sides of their face are a reflection of activity in key parts of the brain that control emotions.

They also say it could because the animals were experiencing conflicted emotions – joy at seeing their owner but sadness at not being able to reach them through the partition.

Dr Nagasawa said: “One explanation for this result is that presentation of the owner elicited both negative and positive emotions.

“A negative response might appear … perhaps because the dogs could not approach and touch their owners despite seeing them through the partitions."

The findings come after psychologists conducted research earlier this year that showed how many owners are able to detect a range of emotions in the facial expressions of their pets.